This invention relates to a method for the reactive dehydrating of manganese dioxide and the application of that technique to prepare manganese dioxide electrodes completely free of water for use in lithium batteries.
There has long been a need for high power density batteries, those having high ionic conductivity electrolytes, resulting in low internal resistance, low viscosity and stability with respect to the electrodes. Lithium batteries have met that need to a certain degree.
These batteries, however, present certain problems because of the high reactivity of lithium with water. The presence of any water in the cell with lithium results in the corrosion of the lithium and raises the possibility of the cell overheating, rupturing or even exploding. Certain measures have alleviated that problem somewhat. Non-aqueous electrolytes are often used with batteries of this type. Precautions are taken to dry all cell components thoroughly before they are to be assembled in the cell. Further, components are assembled in an anhydrous environment to ensure that they are not recontaminated by the moisture found in room atmosphere.
Manganese dioxide has been used for a long time as an active cathode material in alkaline dry batteries. In the 1970's, it was established that the lithium cell could be put into practical use by employing almost anhydrous manganese dioxide as cathode material for non-aqueous lithium cells. Because of its physical, chemical and electrical properties, the manganese dioxide has great oxidizing power and therefore high depolarizing characteristics. These characteristics make it an excellent choice for high performance dry batteries.
Commercially available, manganese dioxide, however, is not a completely dry material. In the past, a heat treatment has been used to remove the combined water (water of crystallization). This heat treatment is done in air or in a vacuum at various temperatures for as long as a day or longer. The heat treatment must be done at a minimum of 250.degree. C. but it is generally hypothesized that all water, both adherent and combined, is not removed until a temperature of 750.degree. C. is reached. Yet, for practical considerations, it was felt that the presence of a little water was unavoidable when manganese dioxide was used, because it was bound in the crystal structure. Its presence can even be detected on X-ray diffraction indices. It was felt that because the water was bound in the crystal structure, it would have no effect on storage characteristics. However, lithium-manganese dioxide batteries, despite their excellent short term performance, have not performed as expected because they have not demonstrated stability in storage.